Organic polymers are used for many optical applications, the largest being in the area of consumer eyewear. In this application, organic polymers afford a variety of properties including low density and resistance to impacts and scratches. Organic polymers may also be processed into what are referred to as progressive lenses that have a graded refractive index to accommodate both near-sighted and far-sighted corrections. The organic polymers typically used in contemporary eyewear applications are commonly, but not exclusively, composed of polymethracrylates, polycarbonates, or polythiourethanes. The optical parameters of importance in the choice of polymer for this application are the refractive index, Abbe number, and optical clarity. Materials currently in use typically are in the refractive index range (“R.I.”) of 1.65-1.68, though materials with R.I. of 1.70 or above have been synthesized. The Abbe number represents the chromatic dispersion of light, which is the refractive index, measured at specific wavelengths (specifically the Fraunhofer d, f, and c wavelengths). The Abbe number is inversely proportional to the chromatic dispersion. It generally varies between 60, which translates to a very low chromatic dispersion and 30, which is a highly chromatic material. For eyewear applications, the Abbe numbers in the range of 35-40 are acceptable and are typical for the organic polymers used.
For optical applications in general and eyewear in particular, the synthesis of new polymers with refractive indices >1.65 and acceptable Abbe numbers is of considerable importance. Higher refractive index materials will permit smaller, lighter weight lenses to be used and provide a much broader graded index for progressive lenses. The material modification that leads to higher refractive indices is the incorporation of highly polarizable atoms and ions. Incorporating such polarizable groups has been the standard protocol used to develop new high R.I. polymers. The electronic polarizability is a tensor property of an atom or molecule that measures the distortion of the electron cloud in the presence of an applied electric field (which can be an optical field). The more the electron cloud can be distorted, the higher the refractive index. The characteristics of atomic and molecular electronic structure that yield large polarizabilities are well understood and can be predicted from basic chemical principles. In particular, the more electronegative an atom is the less polarizable it will be, hence late first-row elements such as F, O and N tend to yield lower refractive index materials. Better choices are 2nd, 3rd or 4th row main group elements such as S (which is currently used in order to increase the refractive index in many polymeric materials), P, and Sn. From a molecular standpoint, the higher electronegativity of the first row can be overcome by delocalization of the electrons across several atoms. Aromatics are more polarizable than saturated hydrocarbons and compounds such as propylene carbonate and dimethylformamide have high dielectric constants.